The present invention relates to the generation of high voltages and is particularly concerned with the generation of low frequency, high voltages for insulation test purposes.
Low-frequency, high voltage testing of component and machine insulation has been widely used since the basic technique was suggested some 15 years ago. Its principal advantage is that, when testing highly capacitive components, the size of the power supply is much reduced as compared with that required for conventional power frequency test purposes. The tests usually associated with this technique are either simple overpotential tests where the machine or component under test is subjected to very high potentials to check for insulation breakdown, or so-called partial discharge tests which are designed to detect internal discharges which do not actually bridge the electrodes of the component or machine, for example, discharges in cavities in a dielectric, surface discharges along an insulator, and corona discharges around a sharp edge.
The generators at present available for producing such low frequency, high voltages are large, cumbersome and not easily transportable. They have all incorporated some form of mechanical device which has been used either to modulate a fixed frequency signal or to reverse the polarity of a modulated signal. Such generators have therefore had the often severe disadvantages of being mechanically and electrically noisy and also of requiring frequent mechanical maintenance. Furthermore, the generation of electrical noise as a result of the use of such mechanical devices has been found to render the known generators rather unsuitable for discharge detection tests since the latter require clean test signals in order to provide reliable test information.
Further disadvantages of the known generators are that they usually operate either at a fixed frequency or over a very limited frequency range up to a maximum of approximately 5 Hz., and, because of the presence of mechanically operating devices, often produce an output signal which is distorted in the cross-over regions near zero voltage.